It has been known in the prior art that unsaturated hydrocarbons may be obtained from the dehydrogenation of dehydrogenatable hydrocarbons. The dehydrogenation may be effected by subjecting the dehydrogenatable hydrocarbons to a dehydrogenation process at dehydrogenation conditions in the presence of certain catalytic compositions of matter which possess the ability to dehydrogenate said compounds with the resultant formation of olefinic hydrocarbons. The particular dehydrogenation catalysts which are employed are well known in the art and comprise such compounds as nickel composited on a solid support such as diatomaceous earth, kieselguhr, charcoal and iron composited on the same supports, etc.
Other dehydrogenation processes have employed, in addition to the dehydrogenation catalysts, an oxidation catalyst in the reaction process. The presence of the oxidation catalyst is necessitated by the fact that it is advantageous to oxidize the hydrogen which is produced by contact with an oxygen-containing gas in order to maintain the desired reaction temperature. For example, styrene, which is an important chemical compound utilized for the preparation of polystyrene, plastics, resins or synthetic elastomers such as styrene-butadiene rubber, etc., may be prepared from the dehydrogenation of ethylbenzene. The dehydrogenation of ethylbenzene into styrene, which is effected by treating ethylbenzene with steam in the presence of a modified iron catalyst, is endothermic in nature. The heat of reaction is about 30 Kcal per mole of ethylbenzene. Therefore, the temperature of the catalyst bed decreases significantly during the progress of the reaction in a commercial adiabatic reactor resulting in limitation of ethylbenzene conversion to a low level. The limitation of conversion arises from the fact that the equilibrium conversion of ethylbenzene is lowered and the rate of ethylbenzene dehydrogenation decreases as the reaction temperature decreases. The decrease of temperature adversely affects not only the conversion level, but also the selectivity for styrene, since at equilibrium conditions, only undesirable side reactions continue to take place. Therefore, it is necessary to maintain the desired temperature level in order to provide a high equilibrium conversion level and a high reaction rate. In the conventional process, the maintenance of temperature is attained by reheating the product stream with the addition of superheated steam between dehydrogenation catalyst beds using a multicatalyst bed reactor system. However, consumption of the additional superheated steam is considerably high and makes the dehydrogenation process costly. Accordingly, significant process economic improvements over the conventional ethylbenzene dehydrogenation processes can be achieved if the reaction temperature is somehow maintained while eliminating or reducing the additional superheated steam. One method of providing for the maintenance of the reaction temperature is to introduce oxygen into the reaction mixture by way of oxygen or an oxygen-containing gas such as air which will burn the hydrogen formed during the dehydrogenation reaction, this combustion resulting in an exothermic reaction which will provide the necessary amount of heat and, in addition, will shift the equilibrium toward production of styrene since the hydrogen formed in the dehydrogenation is consumed. Consequently, a higher conversion and high styrene selectivity are achievable.
The combustion of hydrogen with the oxygen in the oxygen-containing gas requires the presence of an oxidation catalyst. There are some key requirements for the oxidation catalyst to be useable for such a purpose. The most important catalytic property required is good catalytic stability since the oxidation catalyst must survive under very severe reaction conditions, namely at about 6000.degree. to 650.degree. C. in the presence of steam. Under such conditions, porous inorganic materials such as aluminas, silicas and zeolites cannot maintain their pore structures for a long period of time, resulting in the permanent damage of catalysts prepared using such materials as supports, e.g., platinum supported on a porous high surface area alumina, silica, or zeolite. Secondly, the oxidation catalyst must be very active to achieve complete conversion of oxygen to avoid poisoning of iron-based dehydrogenation catalysts which are sensitively oxidized with oxygen to lose their dehydrogenation activities. Thirdly, the oxidation catalyst must be selective for oxidation of hydrogen. Otherwise, ethylbenzene and styrene are consumed to lower the efficiency of styrene production.
Various U.S. patents have described types of oxidation catalysts which may be employed in this process. For example, U.S. Pat. No. 3,437,703 describes a catalytic dehydrogenation process which employs, as a dehydrogenation catalyst, a composition known in the trade as Shell-105 which consists of from 87% to 90% ferric oxide, 2% to 3% chromium oxide, and from 8% to 10% of potassium oxide. In addition, another dehydrogenation catalyst which is employed comprises a mixture of nickel, calcium, chromic oxide, graphite with a major portion of a phosphate species. In addition to these dehydrogenation catalysts, the reaction also employs a catalyst for the oxidation step of the process comprising platinum or palladium in elemental form or as a soluble salt. Another U.S. Pat. No. 3,380,931, also discloses an oxidation catalyst which may be used in the oxidative dehydrogenation of compounds such as ethylbenzene to form styrene comprising an oxide of bismuth and an oxide of a metal of Group VIB of the Periodic Table such as molybdenum oxide, tungsten oxide or chromium oxide. In addition, the patent also states that minor amounts of arsenic may also be present in the catalytic composite as well as other metals or metalloids such as lead, silver, tin, manganese, phosphorus, silicon, boron and sulfur.
U.S. Pat. No. 3,855,330 discloses a method for the production of styrene in which ethylbenzene is treated in the vapor state by passage over a dehydrogenation catalyst and an oxidation catalyst while introducing oxygen into the reaction medium. The dehydrogenation catalysts which are employed are those which have been set forth in various prior U.S. patents and which may be similar in nature to the dehydrogenation catalysts previously discussed. The types of oxidation catalysts which may be employed will include platinum or palladium catalysts which are composited on alumina or molecular sieves zeolite-type which have been charged with ferrous, heavy or noble metals. The patent lists the types of catalysts which are employed including copper or various zeolites, platinum on alumina, platinum on spinel, platinum and sodium on zeolites, platinum, sodium and potassium on zeolites, etc.
U.S. Pat. No. 3,670,044 discloses a method for dehydrogenating cycloalkane, arylalkane and alkanes in the presence of gaseous hydrogen or mixture of gaseous hydrogen and gaseous oxygen using a catalyst composition comprising a Group VIII metal or a mixture of a Group VIII metal and a Group IVA metal deposited on a support comprising a Group II aluminate spinel. It is noted that the patentee teaches that added hydrogen is used in connection with the oxygen, and that when only oxygen is used, the conversion and selectivity are generally low. The addition of hydrogen is believed to be a significant disadvantage in the dehydrogenation process inasmuch as the equilibrium conversion is lowered. This is in contradistinction to the process of the present invention wherein the dehydrogenation process, prior to the oxidation step, is not effected in the presence of any added hydrogen. As will hereinafter be shown in greater detail, the present process results in the selective oxidation of hydrogen with a concomitantly lower selectivity to carbon monoxide and carbon dioxide. In addition, the patentee teaches the use of one catalyst for both dehydrogenation and oxidation which is in contrast to the separate dehydrogenation and oxidation catalysts which are used in the present process.
In addition to the aforementioned U.S. patents, another patent, namely U.S. Pat. No. 4,435,607 also discloses a method for the dehydrogenation of dehydrogenatable hydrocarbons utilizing a two-step process which includes dehydrogenation followed by a selective oxidation process. The catalyst which is employed for the selective oxidation will comprise a noble metal of Group VIII, a metal of Group IVA and, if so desired, a metal of Group IA or IIA of the Periodic Table composited on a highly porous inorganic support. The catalyst base which is used to prepare the oxidation catalyst in this patent was prepared from particles which were relatively small in size.
The patent utilizes as a support for the catalytic metals an alumina which does not possess the properties which are possessed by the precursor or the calcined alumina of the present invention and is totally silent with regard to pore distribution and density of the support. In other words, the patent does not teach the desirability of utilizing as an alumina support for the oxidation catalyst of the process of the present invention a support which possesses the requisite pore size distribution.
However, as will hereinafter be shown in greater detail, we have now discovered that by utilizing, as a support for the metallic portions of the catalyst, a particular type of alumina precursor which possesses certain physical properties, it is possible to obtain a superior catalyst with relation to stability and conversion over those catalysts which have been used in prior processes.